Purpose: The Mediator complex is a multiprotein assembly, which serves as a hub for diverse signaling pathways to regulate gene expression. Because gene expression is frequently altered in cancer, a systematic understanding of the Mediator complex in malignancies could foster the development of novel targeted therapeutic approaches.Experimental Design: We performed a systematic deconvolution of the Mediator subunit expression profiles across 23 cancer entities (n ¼ 8,568) using data from The Cancer Genome Atlas (TCGA). Prostate cancer-specific findings were validated in two publicly available gene expression cohorts and a large cohort of primary and advanced prostate cancer (n ¼ 622) stained by immunohistochemistry. The role of CDK19 and CDK8 was evaluated by siRNA-mediated gene knockdown and inhibitor treatment in prostate cancer cell lines with functional assays and gene expression analysis by RNAseq.Results: Cluster analysis of TCGA expression data segregated tumor entities, indicating tumor-type-specific Mediator complex compositions. Only prostate cancer was marked by high expression of CDK19. In primary prostate cancer, CDK19 was associated with increased aggressiveness and shorter disease-free survival. During cancer progression, highest levels of CDK19 and of its paralog CDK8 were present in metastases. In vitro, inhibition of CDK19 and CDK8 by knockdown or treatment with a selective CDK8/CDK19 inhibitor significantly decreased migration and invasion.Conclusions: Our analysis revealed distinct transcriptional expression profiles of the Mediator complex across cancer entities indicating differential modes of transcriptional regulation. Moreover, it identified CDK19 and CDK8 to be specifically overexpressed during prostate cancer progression, highlighting their potential as novel therapeutic targets in advanced prostate cancer.

BACKGROUND: Overexpression of the ERG protein is highly prevalent in prostate cancer (PCa) and commonly results from gene fusions involving the ERG gene. Recently, N-terminal epitope-targeted mouse and a C-terminal epitope-targeted rabbit monoclonal anti-ERG antibody (ERG-MAbs) have been introduced for the detection of the ERG protein. Independent studies reported that immunohistochemistry (IHC) with both ERG-MAbs highly correlates with the underlying ERG gene rearrangement status. However, comparative studies of both antibodies are lacking. Here, we are among the first to compare the mouse ERGMAb with the rabbit ERG-MAb for their concordance on the same PCa cohort. Furthermore, we assessed whether the ERG protein expression is conserved in lymph node and distant PCa metastases. METHODS: We evaluated tissue microarrays of 278 specimens containing 265 localized PCa, 29 lymph node, 30 distant metastases and 13 normal prostatic tissues. We correlated ERG protein expression with ERG rearrangement status using an ERG break-apart fluorescence in-situ hybridization assay and IHC of both ERG-MAbs. RESULTS: ERG expression and ERG rearrangement status were highly concordant regardless of whether the mouse or rabbit ERG-MAb was used (97.8% versus 98.6%, respectively). Of interest, both ERG antibodies reliably detected the ERG expression in lymph node and distant PCa metastases, of which a subset underwent decalcification. Lymphocytes only revealed immunoreactivity using the rabbit ERG-MAb. If ERG protein expression was present in localized PCa, we observed the same pattern in the corresponding lymph node metastases. CONCLUSIONS: By demonstrating a broad applicability of IHC to study ERG protein expression using either antibody, this study adds an important step toward a facilitated routine clinical application. Further, we demonstrate that the clonal nature of the ERG rearrangement is not restricted to the genomic level, but proceeds in the proteome. Together, our results simplify future efforts to further eliucidate the biological role of ERG in PCa.

The mediator complex is an evolutionary conserved key regulator of transcription of protein-coding genes and an integrative hub for diverse signaling pathways. In this study, we investigated whether the mediator subunit MED15 is implicated in castration-resistant prostate cancer (CRPC). MED15 expression and copy number/rearrangement status were assessed by immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH), respectively on 718 prostate cancer (PCa) specimens and sequenced by Sanger on a subset. Furthermore, SMAD3 phosphorylation, androgen receptor (AR) and proliferation markers were evaluated by IHC. In PCa cells, siRNA/shRNA knockdown of MED15 was followed by proliferation assays with/ without dihydrotestosterone (DHT), and treatments with recombinant TGF-b3. Our results show that MED15 is overexpressed in 76% of distant metastatic CRPC (CRPC MET ) and 70% of local-recurrent CRPC (CRPC LOC ), in contrast to low frequencies in androgen-sensitive PCa, and no expression in benign prostatic tissue. Furthermore, MED15 overexpression correlates with worse clinical outcome thus defining a highly lethal phenotype. Moreover, TGF-b signaling activation associates with MED15 overexpression in PCa tissues, and leads to increased expression of MED15 in PCa cells. MED15 knockdown effects phosphorylation and shuttling of p-SMAD3 to the nucleus as well as TGF-b-enhanced proliferation. In PCa tissues, MED15 overexpression associates with AR overexpression/amplification and correlates with high proliferative activity. MED15 knockdown decreases both androgen-dependent and -independent proliferation in PCa cells. Taken together, these findings implicate MED15 in CRPC, and as MED15 is evolutionary conserved, it is likely to emerge as a lethal phenotype in other therapeuticresistant diseases, and not restricted to our disease model.

In a recent effort to unravel the molecular basis of prostate cancer (PCa), Barbieri and colleagues using whole-exome sequencing identified a novel recurrently mutated gene, MED12, in 5.4% of primary PCa. MED12, encoding a subunit of the Mediator complex, is a transducer of Wnt/b-catenin signaling, linked to modulation of hedgehog signaling and to the regulation of transforming growth factor beta (TGFb)-receptor signaling. Therefore, these studies prompted us to investigate the relevance of MED12 in PCa. Expression of MED12, SMAD3 phosphorylation, and proliferation markers was assessed by immunohistochemistry on tissue microarrays from 633 patients. siRNA-mediated knockdown of MED12 was carried out on PCa cell lines followed by cellular proliferation assays, cell cycle analysis, apoptosis assays, and treatments with recombinant TGFb3. We found nuclear overexpression of MED12 in 40% (28/70) of distant metastatic castration-resistant prostate cancer (CRPC MET ) and 21% (19/90) of local-recurrent CRPC (CRPC LOC ) in comparison with frequencies of less than 11% in androgen-sensitive PCa, and no overexpression in benign prostatic tissues. MED12 expression was significantly correlated with high proliferative activity in PCa tissues, whereas knockdown of MED12 decreased proliferation, reduced G1-to S-phase transition, and increased the expression of the cell cycle inhibitor p27. TGFb signaling activation associates with MED12 nuclear overexpression in tissues and results in a strong increase in MED12 nuclear expression in cell lines. Furthermore, MED12 knockdown

The roles of different epigenetic factors in numerous cancers have been unraveled recently, leading to the initiation of clinical studies. With inhibitors of BET bromodomain proteins, the histone methyltransferases EZH2 and DOT1L, and the histone demethylase LSD1 progressing through clinical trials, and the recognition of the importance of ncRNAs as potential biomarkers and therapeutics, this bears the hope that novel epigenetic therapies will be approved soon.